This invention generally relates to devices for limiting optical surges, and is specifically concerned with a photothermal signal limiter for limiting an optical continuous wave transmission that exceeds a preselected threshold power level.
Devices for limiting optical surges are known in the prior art. For example, optical fuses are known for preventing the transmission of a potentially damaging optical power surge through a photonic circuit. Such a power surge may be caused, for example, by the creation of a spurious amplitude spike at the interface between the input of an optical amplifier and a photonic channel multiplexer. The amplified spike creates a surge that can permanently damage one or more of the optical components of the amplifier itself, thereby rendering it inoperable. Such surges also have the potential of damaging other optical components downstream of the surge-originating amplifier.
To prevent such damage, it is known to place an optical fuse in the amplifier circuit that will break the circuit upon receipt of a power pulse above a certain threshold power level to thereby protect all of the downstream optical components. Such an optical fuse is disclosed in European Patent No. 943954, and generally comprises a light-absorbing metal (such as finely powdered aluminum, palladium, cobalt, etc.) embedded within a light conductive matrix of silicon dioxide or organic dielectric. In operation, when a surge is transmitted through the fuse, the heat generated by the absorption of the surge by the metal degrades or destroys the surrounding matrix, thus breaking the light conducting path through the optical fibers.
Other types of optical surge limiters are known wherein a material having non-linear optical absorption properties is encased within a light conductive material such as glass or a transparent polymer. Such limiters are designed to protect optical devices used in certain military applications from damage from high energy laser pulses. Examples of such limiters are present in U.S. Pat. Nos. 4,890,075 and 5,741,442, and may comprise a glass cell filled with a liquid solution of a dye having the designated non-linear optical absorption properties.
While both of the previously discussed prior art devices are capable of limiting optical power surges, both have shortcomings when applied to fiber optic circuitry. For example, once an optical fuse is xe2x80x9ctripped,xe2x80x9d it must be replaced since the glass or polymer matrix forming the body of the fuse is destroyed or degraded incident to the performance of its protective function. And while many of the surge limiting devices employing non-linear optical absorption materials are self-healing and reusable, they are not designed to effectively attenuate a power surge of relatively moderate proportions occurring over a continuous wave optical transmission. By continuous wave optical transmission is modulated optical power which has continuous average power over a time scale significantly longer than the communication frequency. By contrast, they can only be actuated by very high amplitude, short duration (i.e. 10 nanoseconds) bursts of laser light. Moreover, they often employ exotic and relatively expensive materials (such as fullerenes modified to have side chains) and require structural components (such as liquid or gel-containing cells formed from glass, quartz, or sapphire) which are difficult to fabricate on the small scales necessary to join optical fibers together, and which require separate joining structures when installed between two optical fibers.
Clearly, there is a need for an optical signal limiter that is reusable, and effective to limit power surges transmitted over continuous wave optical signals. Such a limiter should be easy to mass produce on the small scales used in fiber optic circuits, and formable from relatively inexpensive materials. Finally, it would be desirable if the structure of the surge suppressor were capable of inherently forming a joint or junction between two optical fibers when installed without the need for a separate joining structure or separate joining steps.
According to one aspect of the present invention, the optical signal limiter comprises a limiter body formed at least in part of a material having a negative thermal index coefficient.
In the preferred embodiment, this coefficient is between about xe2x88x920.5xc3x9710xe2x88x924 xc2x0C.xe2x88x921 and xe2x88x924.0xc3x9710xe2x88x924 xc2x0C.xe2x88x921 for an optical signal having a wavelength between about 1400 and 1700 nm. Additionally, the material forming the limiter body has an absorption coefficient of between about 1.0 to 5.0 dB/cm. The limiter body includes input and output ends for receiving and transmitting an optical signal, respectively. The distance between the input and output ends is preferably between about 50 and 1500 microns, and more preferably between 100 and 1000 microns.
The material forming the limiter body is preferably a polymer with adhesive characteristics capable of inherently forming a bond or splice between optical fibers when installed between two fiber ends. The polymer preferably has rubbery or gel-like mechanical properties at the operating temperature of interest. The polymeric material may be ultraviolet curable to facilitate the manufacturing process. Examples include epoxies, acrylates, urethane acrylates, or thiol-ene polymers. Two-part thermally curable or room curable polymers such as epoxy-amine adhesives may also be used. A dopant may be intermixed with the epoxy polymer in order to enhance absorption of the optical limiter in the telecommunication wavelength region (980-1620 nm). In the preferred embodiment, such dopants include rare earth organic complexes.
The signal limiter may also include a pair of first and second collimating fibers to minimize signal loss across the distance between the input and output ends of the body of the limiter. Each collimating fiber includes an end optically connected to an optical fiber, and an opposite end optically connected to one of the input and output ends of the body of the limiter. The collimating fibers may be either a gradient refractive index lens or a thermally expanded core collimating lens. An alignment means such as a fiber gripper, V-groove, or microcapillary tubes may be used to align the cores of the first and second collimating fibers across the gap occupied by the limiter body.
The specific photothermal attenuation properties of the resulting limiter may be conveniently selected by adjusting (1) the negative thermal index coefficient of the material by lowering the glass transition temperature of the limiter body to a value below the environmental temperature the limiter will be operating in, (2) adjusting the length of the gap between the collimating fibers that is filled by the limiter body, and/or (3) adjusting the absorption coefficient of the limiter body by selecting a polymer material with greater or lesser absorption characteristics and/or adding a dopant that will enhance the absorption characteristics. Forming the body of the limiter from a curable epoxy polymer advantageously obviates the need for separate joining materials or a separate joining step during the manufacture and installation of the limiter in an optical circuit.